A few minor changes since v2.2.13.rc1, mainly making the Pigeonhole tests pass.

* Fixed a DoS attack against imap/pop3-login processes. If SSL/TLS handshake was started but wasn't finished, the login process attempted to eventually forcibly disconnect the client, but failed to do it correctly. This could have left the connections hanging around for a long time. (Affected Dovecot v1.1+)
+ mdbox: Added mdbox_purge_preserve_alt setting to keep the file within alt storage during purge. (Should become enforced in v2.3.0?)
+ fts: Added support for parsing attachments via Apache Tika. Enable with: plugin { fts_tika = [tikahost:9998/tika/](http://tikahost:9998/tika/) }
+ virtual plugin: Delay opening backend mailboxes until it's necessary.
This requires mailbox_list_index=yes to work. (Currently IMAP IDLE command still causes all backend mailboxes to be opened.)
+ mail_never_cache_fields=* means now to disable all caching. This may be a useful optimization as doveadm/dsync parameter for some admin tasks which shouldn't really update the cache file.
+ IMAP: Return SPECIAL-USE flags always for LSUB command.
- pop3 server was still crashing in v2.2.12 with some settings
- maildir: Various fixes and improvements to handling compressed mails, especially when they have broken/missing S=sizes in filenames.
- fts-lucene, fts-solr: Fixed crash on search when the index contained duplicate entries.
- Many fixes and performance improvements to dsync and replication
- director was somewhat broken when there were exactly two directors in the ring. It caused errors about "weak users" getting stuck.
- mail_attachment_dir: Attachments with the last base64-encoded line longer than the rest wasn't handled correctly.
- IMAP: SEARCH/SORT PARTIAL was handled completely wrong in v2.2.11+
- acl: Global ACL file handling was broken when multiple entries matched the mailbox name. (Only the first entry was used.)